HD600s are no doubt very good headphones - but it depends on your preferences: if 'soundstage' is of importance to you, the Hd600s are not the first choice - it's relatively narrow.
Sundaras' soundstaging seems to be clearly 'better':
"The Sennheiser HD 600 and the HiFiMan Sundara are both impressive audiophile headphones, though they have slightly different sound profiles. The HiFiMan produce more thump and rumble while slightly reducing the intensity of vocals and instruments. The Sennheiser sound a bit more bass-light in comparison, and they put more emphasis on vocals and instruments. The HiFiMan have a more natural soundstage and are much more comfortable, but there have been reports of quality control issues from the brand."
https://www.rtings.com/headphones/t...iman-sundara/325/589?usage=9287&threshold=0.1
Read the whole comparison.
The HD560s also seem to be better in that regard. People are divided with regard to their sound quality (see below).
Fidelio X2Hr would be a 'cheapo' alternative, with very good build quality; open spacious soundstage and, they are, generally, a good sounding alrounder (when eq'd to Oratory). Vocals maybe hissing on some recordings. (Note: genuin earpads no more available).
"The Philips X2HR have a very good passive soundstage. Thanks to their open-back design, their soundstage is very spacious and open and music is perceived as being in front of you as opposed to inside your head."
https://www.rtings.com/headphones/reviews/philips/fidelio-x2hr
With regard to the 560s , Crinacle is more skeptical, and is favouring the HD600s - but, of course, we're in the realm of subjective impressions:
"The closest thing in tuning to the HD560S within the Sennheiser lineup, in my opinion, would be the HD600. It’s far less warm than the HD650 and, while the upper-mid/treble emphasis does evoke a sense of “HD800ish-ness” due to its intensity and relative forwardness, I would say that the overall tonality is actually better done on the HD560S.
But still, going from the 560S to the 600 is like milk after a spicy curry. The HD600 still remains as one of my tonal benchmarks for headphones, and highlights the little quirks that would make me avoid the HD560S as a true daily driver."
Also with regard to imaging:
"Imaging: a little weird. Vocals are up front and centre all the time and instruments tend to blend together despite the wide stage they are placed in."
https://crinacle.com/2020/11/06/sennheiser-hd560s-review-the-evolved-500/